Aluminum reduction cells or pots traditionally are controlled manually. One of the more important actions required of the operator is to initiate the feeding of alumina to the cell, with considerable judgment and background knowledge being a prerequisite on the part of the operator. Operating efficiency of the cells depend upon the percent alumina in the electrolyte, which normally ranges between approximately 1% and approximately 8%.
Industrial cells change their mode of operation when the alumina content falls to approximately 11/2%, at which time an "anode effect" occurs. This anode effect requires voltage as much as 10 times normal operating voltage to force the pot having the anode effect to pass normal amperage of the potline. In a series of electrolytic cells in which there is a limitation on line voltage, an anode effect on one pot means that the voltage on the remaining pots is decreased and therefore the line current decreases slightly, for example, 2%. If several reduction cells in the line are having anode effects simultaneously, the line amperage can fall as much as 10% below normal operating levels. This reduced amperage means that the remaining cells are not producing as much aluminum as they would if the amperage were at a level which is possible when there are no anode effects in the line.
Since anode effects mean lost production and higher cost in the reduction plant, it is customary to feed alumina to the cells on a schedule such that anode effects do not occur more often than about one per day per pot. In some cell types, it is possible to suppress the anode effect even longer by scheduled feeding of alumina. The closer the operator comes to complete suppression of the anode effect, however, the closer he comes to overfeeding, which results in excessive undissolved alumina in the electrolyte. Eventually, if overfeeding is sustained, the pot becomes "sick", a condition also causing a loss of production and additionally resulting in excessive heating in the cell.
One method of determining alumina concentration is to take samples of the bath and to run chemical analysis. Disadvantages of this method are that it requires considerable cost and time during which the pot, if close to the lower limit of alumina concentration, may produce an anode effect.
It has long been desired by operators engaged in alumina reduction involving the electrolytic process to have an instrument system which would permit the direct and quick determination of the percent alumina dissolved in the cryolite electrolyte. In U.S. Pat. No. 3,471,390, the disclosure of which is hereby incorporated herein by reference, such an alumina concentration metering system is disclosed. The system includes a power supply connected to an immersible probe having anode and cathode members, means for applying voltage of increasing magnitude to the probe and means for indicating the alumina concentration of the cell corresponding to the occurrence of an abrupt increase in the resistance between the anode and cathode members of the probe, an anode effect.
While this alumina concentration metering system is effective, problems have been noticed with regard to the probe member employed therein. First, there is a relatively large distance between the active anode and cathode surfaces, which distance is increased by the fact that the anode and cathode surfaces do not lie in a common plane, requiring electrical current to flow "around a corner" from the anode to the cathode. Further, assembly problems resulted from the need to employ either a carbonaceous or a refractory cement to construct the probe in such a manner that the various components thereof, when fitted to one another, would be "bath tight" to the highly penetrating molten cryolite-alumina electrolyte bath.
It has also been found desirable to be able to determine the temperature of the bath in conjunction with the measurement of its alumina content.